1. Field of the Invention
This invention relates to nuclear well logging apparatus and methods for determining the nature of fluids in formations through which a well bore is formed. More particularly, the invention relates to determining the hydrocarbon saturation (or its correlative, water saturation) of formations adjacent a well bore by nuclear radiation logging. Still more particularly, the invention relates to inelastic gamma ray spectrum logging of a formation with correction for gamma rays from borehole fluids.
2. Description of Prior Art
A major goal of well logging is to establish the fraction of pore space occupied by hydrocarbons. Three methods of doing so have been developed in the prior art. Electrical resistivity and thermal neutron decay methods measure the water saturation S.sub.w, and the difference, S.sub.o =1-S.sub.w is then the saturation of all other liquids and gases. (The term S.sub.o, or "oil" saturation will be used hereafter to refer not only to liquid hydrocarbons, but also to gaseous hydrocarbons). Both the electrical resistivity and thermal neutron decay methods depend upon the presence of salts dissolved in the water and for that reason are less effective in fresh water than in salt water environments.
Because hydrocarbons contain carbon and water contains oxygen, methods and apparatus have been developed for detecting carbon and oxygen and other elements with a logging sonde. When a high energy ("fast") neutron is scattered inelastically from carbon, a 4.438 MeV gamma ray is emitted; when a neutron is scattered from oxygen, a 6.1 MeV gamma ray is emitted. Therefore, logging apparatus which counts the number of 4.438 MeV gamma rays and the number of 6.1 MeV gamma rays and determines their ratio should be under ideal conditions, a measure of the ratio of carbon to oxygen in the formation. Such measurements are known in the art as carbon/oxygen or simply, C/O measurements or C/O logs. Moreover, a calcium/silicon ratio can also be obtained. Comparison of the two ratios permits the user to distinguish carbon in calcium carbonates from that in hydrocarbons.
In cased hole wells, where the salinity or salt content of a water-saturated zone is not known, is very low, or has been altered by production by water injection, the C/O measurement is the only alternative to resistivity and neutron decay methods.
Under actual field conditions, however, the well bore may contain hydrocarbons (in the form of oil or gas) and water. Consequently, C/O measurements of the formation are contaminated or "corrupted" with gamma rays resulting from the collision of fast neutrons with carbon and oxygen atoms of fluids in the borehole. In addition, clays of certain earth formations contain carbon atoms. Such contamination of the inelastic gamma ray spectral data, and ultimately of the S.sub.o determination, may be eliminated if the porosity, lithology, borehole configuration and hydrocarbon content of the fluid in the well bore is known with precision.
Characteristics as a function of depth of a cased well, such as porosity and lithology of the formation and the borehole configuration may be known. But C/O logging has been highly sensitive to uncertainty of the borehole oil/water mixture. For this reason C/O logging measurements of cased, producing wells have required that the well be "shut-in" so that the borehole fluid components may be known better. However, even with shut-in wells, the content of borehole fluid is not always known well enough.
U.S. Pat. No. 4,507,554 issued Mar. 26, 1985 to Hertzog and Nelligan describes a nuclear logging method by which a pulsed neutron source and a single detector comprising a scintillation crystal and a photomultiplier tube are used to detect spectra of signals resulting from a neutron pulse. An inelastic spectrum, an early capture spectrum and a later capture spectrum are measured. The early capture and late capture spectra are compared to derive a representation of the spectral composition of the borehole contents. The inelastic spectra having gamma ray counts of borehole and formation fluids and matrix, is corrected using the desired spectrum of the borehole contents.
U.S. Pat. No. 4,645,926 issued Feb. 24, 1987, to Randall describes a nuclear logging system and apparatus having a pulsed neutron source and near and far detectors. Count rate data are accumulated over a plurality of discrete intervals covering the entire interval during and between bursts of the neutron source to obtain both inelastic and capture portions of the detected gamma rays. An example of the use of the inelastic portion of the spectrum is to generate parameters highly sensitive to changes of the borehole, because of the shallow depth of investigation available from the inelastic radiation. The borehole conditions most evidenced by such parameters are those related to changes in the geometry of the borehole. A ratio of total counts of near detector inelastic gamma rays is used as an indicator of changes in the borehole for diagnosing system irregularities and as an indicator of borehole effects on other parameters.
While the technology of the patents described above represent efforts to advance the nuclear logging art, there has remained the need for a method and apparatus by which the oil saturation S.sub.o of a formation may be accurately determined through C/O logging techniques with correction for corrupting gamma rays of unknown amounts of hydrocarbons in the borehole. Consequently, several objects of the invention have been identified.